Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Applications for fuel cells include battery replacement, mini and microelectronics, car engines, power plants, and many others. One advantage of fuel cells is that they are substantially pollution-free.
In hydrogen fuel cells, hydrogen gas is oxidized to form water, with a useful electrical current produced as a byproduct of the oxidation reaction. A solid polymer membrane electrolyte layer can be used to separate the hydrogen fuel from the oxygen. The anode and cathode are arranged on opposite faces of the membrane. Electron flow between the anode and cathode layers of the membrane electrode assembly can be exploited to provide electrical power. Hydrogen fuel cells are impractical for many applications, however, because of difficulties related to storing and handling hydrogen gas as well as other reasons.
Organic fuel cells can prove useful in many applications as an alternative to hydrogen fuel cells. In an organic fuel cell, an organic fuel such as methanol is oxidized to carbon dioxide at an anode, while air or oxygen is simultaneously reduced to water at a cathode. One advantage over hydrogen fuel cells is that organic/air fuel cells can be operated with a liquid organic fuel. This eliminates problems associated with hydrogen gas handling and storage. Some organic fuel cells, called “indirect”, require initial conversion of the organic fuel to hydrogen gas by a reformer. The required reformer increases cell size, cost and complexity. Other types of organic fuel cells, called “direct,” eliminate these disadvantages by directly oxidizing the organic fuel without conversion to hydrogen gas. To date direct organic fuel cell development has focused on the use of methanol and other alcohols as fuel.
Conventional direct methanol fuel cells have numerous unresolved problems associated with them. For example, methanol and other alcohols have high osmotic and diffusion crossover rates across polymer membrane electrode assemblies. Fuel that crosses over avoids reaction at the anode, cannot be exploited for electrical energy, and thereby limits cell efficiency. Crossover also leads to poisoning of the cathode as fuel crosses over the polymer membrane and blocks reaction sites when it adsorbs onto the cathode catalyst. Efficiency of the cell is thereby further reduced. A proposed solution to this problem has been to provide additional catalyst. The relatively high cost of catalyst particles makes this a costly alternative.
Because of high crossover, methanol and other alcohol fuel cells typically operate with a low fuel concentration of not more than 8%. These relatively low concentrations create additional problems. A supply of ultra-pure water in addition to a water management system that includes at least a sensor, a pump and a filter can be required. This adds cost and complexity, and substantially limits the usefulness of the cells for applications where size and weight become critical such as miniature and microelectronics applications.
Other problems also remain unresolved in the fuel cell arts. For example, so-called passive fuel cells differ from active cells in that passive cells generally have a fixed amount of fuel, whereas active cells have fuel fed to them. Because passive cells are often lower in weight, smaller in size, and otherwise simpler than active cells, they are often favored for mini and microelectronics applications. The efficiency of passive cells can be limited, however, by the circulation of fuel to the anode. If stored fuel cannot be effectively communicated to the anode, it cannot react. Circulation pumps have been proposed to circulate liquid fuel in a passive cell. Such pumps, however, render moot some of the advantages of the passive cell.
Many passive cells, with miniature passive cells being a particular example, require an elevated temperature to produce the level of power density required for many applications. This can be a problem, in that energy must be provided to elevate the temperature. Such cells have proven difficult to use in many near ambient temperature applications, such as battery replacement in miniature and microelectronics devices.
Still other problems in the art relate to methods for making fuel cells and fuel cell assemblies. For example, for miniature and microelectronics applications, fuel cell assemblies that are relatively large, bulky, and/or heavy pose problems. The use of fasteners such as bolts and the like to hold the portions of a cell together, for instance, tend to add size and weight to small scale fuel cells.
These and other problems remain unresolved in the art.